More than 30 former senior U.S. officials and military leaders have urged the White House and Congress to formally approve Montenegro's accession to NATO by 2017, saying the Balkan nation's membership would promote "stability and security in Southeastern Europe."

The signatories of the open letter published on June 20 include former U.S. Defense Secretary Chuck Hagel and two former NATO supreme commanders in Europe, U.S. General Philip Breedlove and Admiral James Stavridis.

In the letter, published by the national-security website War On The Rocks, they said that Montenegro has "built a small but capable military and reformed its security services to meet NATO standards" and "made significant progress" on rule of law, corruption, and organized crime. http://warontherocks.com/2016/03/open-letter-on-donald-trump-from-gop-national-security-leaders/

They also said Montenegro's membership "would clearly reject the notion that any third party" has "a de facto veto on NATO enlargement" -- an apparent reference to Russia's fierce opposition to Podgorica's entry into the alliance.

Montenegro and NATO last month signed a protocol on its accession to the alliance. The nation will participate in NATO meetings as an observer until all member nations ratify the protocol.